The immunopathology of Experimental Autoimmune Myasthenia Gravis (EAMG) in rats involves a neuromuscular impairment caused by a T cell dependent antibody response against the post-junctional acetylcholine receptor (AChR). The result is weakness and rapid fatiguing in the rats similar to symptoms observed in human patients inflicted with myasthenia gravis. However, although anti-AChR antibodies appear to be the direct cause of neuromuscular dysfunction in human patients, it has been observed that there is a disturbing lack of strong correlation between the titer of circulating antibody and the severity of symptoms demonstrated. Thus, the basic premise of this research proposal is that some, but not all, anti- AChR antibodies can cause disease (perhaps related to differences of the exact binding specificity of the antibodies produced). The primary goal of the studies proposed herein is to determine which subsets of anti-AChR antibodies are most responsible for causing EAMG in Lewis rats. Based on information gained in preliminary studies, the identification of these antibody subsets will involve the evaluation of several criteria: 1) Myasthenogenic antibodies are expected to react with AChR of native but not denatured conformation, 2) Myasthenogenic antibodies are expected to be present in EAMG-sensitive Lewis rats but not EAMG-resistant Wistar Furth rats, and 3) Myasthenogenic antibodies are expected to demonstrate binding activity against AChR of muscle origin. The antibody subset that fills these criteria will be distinguished and isolated with the use of specialized anti-idiotypic probes combined with methods of separation involving isoelectric focusing. Confirmation of myasthenogenicity will be performed by intravenous transfer od subseted antibodies into rats followed by direct analysis of AChR-dependent muscle contraction. In addition, antibody-toxin conjugates (immunotoxins) will be prepared that are composed of the A chain of ricin covalently linked to anti-idiotypic antibodies with specificity for idiotypes associated with anti-AChR antibodies showing high disease-causing potential; these conjugates will be tested for their ability to selectively deplete AChR-responsive B cells of their ability to produce the particular antibody specificities identified as myasthenogenic. Information gained in these studies should provide additional needed insights into two issues concerning myasthenia gravis. First, questions involving regulation of the anti-AChR antibody response could be more directly focused on that part of the response directly responsible for disease induction and clear up discrepancies between antibody titer and disease severity. Second, immunotherapeutic strategies designed to eliminate the antibody that causes neuromuscular impairment could be refocused on those anti-AChR antibodies with fine specificities directly responsible for disease.